Massive Dolomitization Along an Upward Branching Fracture System in Middle Triassic Carbonates of the Dolomites, Northern Italy
Edith Newton, Lawrence A. Hardie
The spectacularly exposed Triassic carbonate platforms of the Dolomites, northern Italy, have been studied for almost two centuries, but the origin of the dolomite remains unresolved. In the partly dolomitized Ladinian-age Latemar platform, 5 km across and 1 km high, dolomitization patterns as well as primary sedimentary and early diagenetic features are preserved. Mapping has allowed three-dimensional reconstruction of the dolomite front geometry which records the "frozen" pathways of dolomitizing fluids. Massive alteration of limestone to coarse-grained, vuggy, sucrosic dolomite (at least 10% of the buildup) occurs along an upward and outward branching fracture system associated with nearby late-Ladinian intrusions. A central pipe of dolomite (300 m wide) cuts verticall upward through 400 m of early-cemented subtidal grainstones of the lower platform. The dolomitized zone then expands to a mushroom-like shape which encompasses 1.5 km2 of the overlying cyclical facies (120 m thick) where dolomitizing fluids utilized bedding-planes and uncemented grainstones in addition to fractures. Dolomitization enlarges intergranular pores and alters limestone fabrics to a network of interlocking rhombs (.25-2 mm in size) of zoned, ferroan dolomite. It also obliterates early, fine-grained, selective dolomite in shallowing-upward cycle caps.
The alteration patterns clearly indicate upward moving fluids in the core of the buildup. This suggests circulation of the fluid (the surrounding seawater?) through the fracture and pore network driven by either heat from the late-Ladinian intrusions or Kohout convection. In this regard, later cements of saddle dolomite provide evidence that hydrothermal circulation was active in the Latemar buildup during Ladinian time.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.